Pioneer SA-7800 loing for help - diyAudio
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Old 28th November 2015, 06:06 PM   #1
Depaj is offline Depaj  France
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Default Pioneer SA-7800 loing for help

Hi!

I managed to get hold of great looking pioneer sa-7800 but in not working condition. The output transistors as well as Q24 were blown on one side. On the other side R35 and R59 were shorted and someone replaced R33 and R34 (560ohm each) with 9Mohm ones so I changed all of those. I fired up the amp with, as usual, a 40W light bulb in between. the little flash and then a very light glow as should be, after a couple of seconds the relay clicked. The trouble began when I tried to set the bias, 53mV by adjusting VR5 and 6 and then up to 66mV by turning VR7 and 8. As soon as I started turning VR5 the bulb started glowing way brighter and by the time I got to VR6 (on the other channel) the relay opened up until I got both pots back down. At the same time the vacuum power meter dimmed to almost nothing and a slight noise appeared around the radiator. Even at the bias set at 66mV I have only a few mV of DC before the relay. With the Bias at 0V the amp sounds great and even with one channel up to 66mV (left or right no difference) there is no obvious problem with it.The fault seems to occur when both channels are set at 66mV. All the chemical capacitors are new.

I couldn't find any faulty components and am stuck, if any one has some idea as to what could be wrong I would love to hear (read) it.

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Old 28th November 2015, 06:55 PM   #2
llwhtt is offline llwhtt  United States
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You should be on full AC line voltage when adjusting the bias.

Craig
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Old 28th November 2015, 06:59 PM   #3
Depaj is offline Depaj  France
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Could that really be the only problem ?
I restored A SA 8800 just before and although I did readjust the bias after removing the bulb, I had no trouble adjusting it with it. I will try and let you know.
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Old 28th November 2015, 07:14 PM   #4
Depaj is offline Depaj  France
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I feel kind of stupid now, that solved the problem. I didn't think it could be that simple. Thank you Craig
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Old 28th November 2015, 07:18 PM   #5
llwhtt is offline llwhtt  United States
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The voltages were dropping below normal with all of the current flowing thru the lamp. Live and learn.

Craig
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Old 28th November 2015, 07:26 PM   #6
Depaj is offline Depaj  France
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That sounds very logical indeed, definitely something to keep in mind.
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Old 2nd December 2015, 03:31 PM   #7
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40W is far too low for a bulb tester when you are working with consumer amplifiers in the area 60-100W in class AB 60-100W will be a wise choice Expect more when the amplifier is higher power

You need to recap and also you need to verify that secondary power supply feeding the first stages of the amplifier provides proper and clean voltage .

At this age there is more things to be done while few of them will not be cured with deoxit Relay is to be replaced Phono stage transistors can be also noisy ...

Kind regards
Sakis
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Old 2nd December 2015, 05:25 PM   #8
Depaj is offline Depaj  France
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Thank you for the advise, I'll get myself something in the 60-100w.

The whole amp has had a recap and the power supply has been checked, everything is as it should be.

The phono stage seems ok but I will definitely check the relay.
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Old 2nd December 2015, 05:48 PM   #9
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Very well and properly done 7800 is not famous for it's super power but is a very sweet an nice looking amp

I presume that you checked the capacitors in the VU area ( located upside down easy to miss ) since those get blown very easily

Kind regards
Sakis
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Old 2nd December 2015, 05:54 PM   #10
Depaj is offline Depaj  France
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I didn't get the time to listen to it as I'm not at home right now but I've been using the SA-8800 for a while now and it sounds great although maybe a little "harsh" in the high frequencies.

Yes of course, I checked several times if I didn't miss any.
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